Methods of the type addressed here are used for evaluating capacitive touch or proximity sensors. Such a sensor can detect the presence of an object. Further, with an appropriate design, such as sensor can also detect a touch or the approach by an object, such as a finger or a user or a stylus, within a sensitive area. The touch-sensitive area may be overlaid on a display screen, for example. In one display application, the touch or proximity sensor may allow the user to directly interact with what is displayed on the screen. As such, the user is provided with more than just indirect interaction via a mouse or similar input device.
A number of different types of touch sensors exist. For example, touch sensor types include resistive touch sensors, touch sensors with acoustic surface waves, and capacitive touch sensors. Capacitive touch sensors, which may also be used in particular to detect mere proximity, have become the most widely used.
The value of the capacitance of a capacitive touch sensor changes when an object touches the surface of the sensor or comes into close proximity to the sensor. The task of an associated controller or of a measuring method used by the controller is to process (e.g., measure, detect, analyze) the change in capacitance of the sensor. The measuring method or controller processes the change in capacitance of the sensor in order to detect the triggering touch of the object on the sensor and/or the proximity of the object to the sensor.
A difficulty is that capacitances of capacitive touch sensors, and in particular capacitance changes to be detected, are typically relatively very small. For this reason, so-called integration (or acquisition) processes are often used for measuring the capacitance of a capacitance touch sensor. The integration processes involve transferring small charge quantities in multiple successive so-called integration (or acquisition) cycles from the capacitive touch sensor to an integration capacitor. The value of the capacitance of the capacitive touch sensor is relatively small and variable. The integration capacitor has a known fixed capacitance value. The known capacitance value of the integration capacitor is relatively much larger than the variable capacitance value of the capacitive touch sensor.
DE 10 2010 041 464 A1 (corresponding to U.S. Pat. No. 8,552,994) describes a method for measuring the capacitance value of a capacitive sensor. The measuring method involves an integration process of the type mentioned above. In this integration process a terminal of the capacitive sensor is electrically connected to a first terminal of an integration capacitor at a shared circuit node.
Various methods are used for carrying out the measurement. Thus, for example, after executing a preset number of integration cycles, a voltage is present at the integration capacitor. This voltage of the integration capacitor results from the sum of the charge transfers that have taken place during the integration cycles. After executing a preset number of the integration cycles, the voltage of the integration capacitor is measured and digitized by an A/D converter.
The measured voltage itself, or its digitized value, of the integration capacitor is used as the result of the measurement. Alternatively, a measured value of the capacitance of the capacitive sensor, which is computed from the voltage of the integration capacitor, the known constant capacitance of the integration capacitor, the value of the supply voltage, and/or the number of integration cycles, is used as the result of the measurement.
Alternatively, the voltage of the integration capacitor may be measured after each individual integration cycle. The measurement is ended upon the voltage of the integration capacitor reaching a predefined threshold value. In this case, the measured variable is the number of integration cycles that are executed until the integration capacitor voltage threshold value is reached.
The resolution of these measuring methods, and thus the limit for the distinguishability of two states or capacitance values, is determined essentially by the resolution of the A/D converter used in the integration process. The A/D converter can detect voltages only in specific discrete gradations. These stages are also referred to as quantization intervals. The area to be measured is thus quantized, i.e., divided into discrete regions. In this case, the area to be measured is quantized into voltage stages. During a measurement, the true voltage (i.e., the voltage measured by analog means) is associated with the value of the next higher or next lower stage as the digital measured value, depending on which of these stages is closest to the true voltage. The deviation of the true voltage from the voltage stage output by the A/D converter is the quantization error. Thus, references below to the voltage value measured by the A/D converter mean the digital value of the voltage stage output by the A/D converter.